1. Field of the Invention
This invention relates to electronic ringing generators and more particularly to those ringing generators which must generate a waveform which is close to a pure sinusoid over a wide range of output load conditions including overloads.
2. Description of the Prior Art
The ringing generators used in moderate to large sized telephone systems have typically been provided as modules with output power capabilities ranging from 15 volt-amperes (VA) to 50 VA. They have generally provided sinusoidal, or approximately sinusoidal waveforms, especially those with the larger output capabilities. Such generators usually incorporate relatively large and heavy iron core transformers, and are required to tolerate short circuits on their output terminals without sustaining permanent damage. The expense and size of these generators is tolerable because their cost is distributed among a relatively large number of lines, compared to smaller systems. It is also true that in a large system better advantage can be taken of the fact that the probability that a given line will require ringing at a given instant is quite low. Accordingly, in a large system, the average ringing capacity required per line (total ringing capacity required divided by the total number of lines) is quite small.
In smaller telephone systems, e.g., small loop carrier systems, or perhaps small PBX's or key systems, conditions are quite different. Generally, large expensive ringing systems of the type used in central offices cannot be justified, and more extensive use is made of electronic techniques which do not employ large iron core transformers. In the past, these smaller systems have often employed squarewave ringing, which is inherently more power efficient and easier to realize with electronic techniques. In these smaller systems, it is also required that the capability to ring a large percentage of the lines simultaneously, perhaps all of the lines, be provided.
As these systems proliferated, and as data transmission became more prevalent in the telephone plant, it was observed that the steep rise and fall times associated with squarewave ringing coupled noise into other pairs in the telephone outside plant or inside building wiring, causing both audible noise at the ringing frequency and its harmonics, and impulse noise as well, which interfered with the operation of data circuits. In an effort to mitigate these effects, some manufacturers introduced a few milliseconds of slope in the rising and falling edges of their ringing signals, which did greatly relieve the problem.
In recent years, the number of manufacturers of telephones and other station equipment, and the number of different types of telephone station equipment have greatly increased. The traditional electromechanical ringers once characteristic of telephone sets responded satisfactorily to either squarewave or sinewave ringing. Such is not the case, however, with some of the more modern equipment. Many of the newer devices will not respond reliably to waveforms which are not at least reasonably sinusoidal. Many of the newer devices actually detect sharp rise and fall times and inhibit their alerting devices to prevent them from responding to dial pulses and transients.
As a result of the aforementioned difficulties, squarewave ringing is no longer acceptable in new systems. Ringing waveforms are now controlled in the specifications applicable to such systems by a crest factor requirement. Crest factor is defined to be the ratio of the peak voltage of a waveform to its rms value. A pure sinusoid has a crest factor of 1.41; a pure square wave has a crest factor of 1.00. A common requirement currently imposed by telephone companies is that the crest factor be between 1.20 and 1.60. It may be an objective in such systems that the crest factor be between 1.35 and 1.45. This is intended to ensure that the ringing signal will either be very close to a pure sinusoid or a pure sinusoid.
The introduction of optical fiber into the telephone loop plant greatly increases the need for small, reasonably efficient ringing devices with waveforms characterized by well controlled crest factors; sinusoidal ringing is clearly preferred. Fiber to the curb (FTTC) systems typically serve four (4) residences and provide not more than 12 channels, while fiber to the home (FTTH) systems serve only one residence and seldom provide more than three (3) channels. Each such system must be provided with a ringing source, which must be capable of ringing up to three (3) lines simultaneously, with each line being permitted to have as many as five (5) ringers associated with it. Such a ringing device must be capable of delivering approximately 5 VA of output capability with the required crest factor, and with reasonable efficiency.
The ringing device, in general, along with the other power supplies in the local system, may be powered from a power source which is located as much as 12 kilofeet (about 4,375 meters) away from the local system, and is connected to the power source by a cable pair or pairs. When subjected to transient overloads, the ringing device cannot cause other power supplies in the local system to be deprived of sufficient power to continue operating satisfactorily. Therefore, the total power it can take from the power source must be limited, and of course, the device must be capable of being subjected to a short circuit or low resistance fault on its output without sustaining permanent damage. If subjected to an overload due to an excessive number of ringers being rung simultaneously, it must continue to meet its crest factor requirements, even though it is not required to deliver sufficient voltage to ring the excessive ringer load.
To the end of overcoming the aforementioned difficulties associated with the provision of ringing capability in small local telephone systems such as FTTC and FTTH systems, it is an object of the invention to provide a small, reasonably efficient ringing device characterized by a well controlled output crest factor under a wide range of output load conditions. It is a further object of the invention to provide an output current limiting function for the device, which simultaneously preserves the desired crest factor, and allows the device to continue to operate efficiently while such current limiting function is active. It is yet a further object of the invention to make the device capable of being subjected to short circuit or low resistance output faults without sustaining damage. It is yet a further object of the invention to limit the input power taken by the device under overload conditions so that overloads do not result in the malfunction of other power supplies in the local system which are fed from the same power source.